Need alternative for getting user input

Question

I am building a windows app and I have a menu item that will process a file. However before I get to that file I need 1 piece of data from the user (not the file) and I don't want to create a whole new form for a pop-up dialog box, but are there any alternatives?

I don't think there is, but ideally I want something like messagebox that allows users to enter an alphanumeric code.